1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium which can reversibly perform a coloring reaction and an erasure reaction by applying heat to a reversible thermosensitive coloring material including an electron donating coloring compound and an electron accepting compound while controlling the heat. The present invention further relates to an information storage material comprising an information recording unit and a reversible display unit including the reversible thermosensitive recording medium. The present invention still further relates to a reversible thermosensitive recording label comprising the reversible thermosensitive recording medium and an adhesive layer. The present invention also relates to an image processing method of recording and/or erasing an image in the reversible thermosensitive recording medium and an image processing device comprising a device for recording an image in the reversible thermosensitive recording medium and/or a device for erasing an image therein.
2. Discussion of the Background
Thermosensitive recording media utilizing the coloring reaction between an electron donating coloring compound (hereinafter referred to as a coloring agent or a leuco dye) and an electron accepting compound (hereinafter referred to as a developing agent) are widely known. With the progress of office automation in business environments, these thermosensitive recording media are popularly used in a variety of applications such as output paper for facsimile apparatus, word processors, scientific measuring instruments, etc., and currently magnetic thermosensitive cards such as prepaid cards, reward cards, etc. The conventional thermosensitive recording media actually used are of an irreversible type in which a colored image cannot be discolorized (i.e., decolored). Namely, new information can be written in only non-recorded areas because images once recorded in the media cannot be erased. Consequently the information recording capacity of such conventional thermosensitive media is limited and it is necessary to reduce the amount of information to be recorded and to replace the conventional thermosensitive medium with a new card when the recording capacity thereof is used up. Therefore, considering the environmental problems such as the waste problem and deforestation which are now of great public interest, a need exists for rewritable reversible thermosensitive recording media to reduce the consumption amount of these conventional thermosensitive media.
A variety of reversible thermosensitive recording media have been proposed based on this need. Unexamined published Japanese Patent Applications Nos. (hereinafter referred to as JOP) 63-107584 and 4-78573 disclose reversible thermosensitive recording media of a high molecule type using a change in physical state between transparency and white turbidity. Reversible thermosensitive recording media of a dye type using a chemical reaction have been now disclosed. For example, JOP 60-193691 discloses a developing agent comprising gallic acid and fluoroglucinol, JOP 61-237684 discloses the use of a compound such as phenolphthalein or thymolphthalein as a developing agent, and JOPs 62-138556, 62-138568 and 62-140881 have disclosed a reversible thermosensitive recording layer containing a homogeneous mixture of a coloring agent, a developing agent and a carboxylic acid ester. Further, JOP 3-173684 discloses the use of an ascorbic acid derivative as a developing agent, and JOPs 2-188293 and 188294 have disclosed the use of a salt of a higher fatty amine and gallic acid or bis(hydroxyphenyl)acetic acid as a developing agent.
Further, JOPs 5-124360, 6-210954 and 10-95175 have disclosed reversible thermosensitive recording media including a recording layer comprising thermosensitive coloring material including a leuco dye serving as a coloring agent and a developing agent such as an organic phosphoric acid compound, an aliphatic carboxylate compound or a phenolic compound, each of which has a long chain aliphatic group. The thermosensitive coloring material can achieve a coloring state when heated to a first temperature and the color can be stably retained when rapidly cooled down to room temperature. Further, the colored image can be erased when heated to a second temperature which is lower than the first temperature and the decolorization state is stably retained when cooled down to room temperature. In addition, these coloring state and discolorization state can be repeatedly achieved.
However, these thermosensitive recording media are not satisfactory in terms of coloring sensitivity and image density and thus need improvement. Several countermeasures to improve coloring density and coloring sensitivity have been disclosed and one of the countermeasures is to provide an intermediate layer between a substrate and a reversible recording layer. The intermediate layer has a thermal insulation effect. JOP 2003-11514 discloses the use of an intermediate layer using a hollow silica, JOPs 6-340174 and 8-183254 have disclosed the use of an intermediate layer comprising fine hollow particles comprising a styrene-acrylic copolymer, and JOPs 7-228250 and 7-257036 have disclosed the use of an intermediate layer prepared using a latex including a hollow copolymer containing a carboxylic group. Further, Japanese Patent No. 3007899 discloses the use of an intermediate layer comprising polyvinylidene chloride hollow particles or porous aluminosilicate.
These countermeasures are developed to improve the efficiency of heat applied to the reversible thermosensitive recording layer by providing a layer comprising hollow particles between the substrate and the reversible thermosensitive recording layer. It is recognized that image density can be improved to a certain degree by such a layer. However, the hollow particles used in the countermeasures are limited to particles having a low hollow ratio or a large particle diameter due to the selection restriction on the materials and the methods. Therefore, hollow particles having a high hollow ratio and a small particle diameter have not been used. An intermediate layer comprising particles having a low hollow ratio (e.g., an intermediate layer using the styrene-acrylic fine hollow particles having a hollow ratio of about 50% described in JOPs 6-340174 and 8-183254) has little thermal insulation effect and cannot satisfactorily improve image density. On the other hand, in the case of an intermediate layer comprising hollow particles having a large diameter (e.g., the intermediate layer using vinylidene chloride hollow particles having a particle diameter of about 20 μm disclosed in Japanese Patent No. 3007899 and the intermediate layer using hollow silica particles having a particle diameter of about 40 μm disclosed in JOP 2003-11514), the particle diameter of the hollow particle is relatively large compared with the thickness of the intermediate layer and therefore the surface thereof has a concavo-convex surface. When a reversible thermosensitive recording layer is accumulated on a convex portion of the intermediate layer surface, the reversible thermosensitive layer may not be formed, which leads to a problem such as whiteout in a solid image. Therefore, these countermeasures are also not satisfactory with regard to improvement in image density.
When an erasure method using a heat roller is used for the reversible thermosensitive medium mentioned above, the erasure density has no practical problem. There is another erasure method using a thermal head. The erasure method using a thermal head has an advantage in that its energy consumption is small. However, the erasure density level is not practically satisfying in most cases for the erasure method using a thermal head. Therefore, there is a strong demand for improving the erasure density level by a thermal head erasure method. In addition, the erasable energy-range is not currently satisfactory for this thermal head erasure method. Therefore it is important to enlarge the erasable energy range. Because of these reasons, a need exists for a reversible thermosensitive medium which can exhibit an image having a high coloring image density and good uniformity without causing the whiteout problem, etc., and which has a wide erasable range such that a recorded image can be erased at a good erasure density level by a thermal head erasure method.